In the production of hydrogen gas from hydrocarbons, i.e. the conversion of normally liquid or gaseous hydrocarbons by cracking to products of lower molecular weight, two general processes have been widely employed in the art; (1) a process in which hydrocarbons are mixed with an oxidizing agent to effect partial oxidation, and (2) a process in which hydrocarbons and steam are heated together in a process known as steam reforming.
The process (1) is applicable to a wide range of hydrocarbon feeds, ranging from methane to distillation residue from mineral oils. It has, however, the distinct disadvantage that substantially pure oxygen is required as the only practical oxidizing agent.
In process (2), even though pure oxygen is not required for cracking, the range of hydrocarbon feed is severely limited. This is because the process is markedly sensitive to carbon deposition. This tendency toward carbon deposition increases as the molecular weight of the starting feed is increased, and is especially marked during the use of the conventional nickel oxide catalyst. Hence, only hydrocarbons of low molecular weight can be used in (2), and industrial utilization of hydrocarbons heavier than naphtha (light oil) has not been feasible.
Another problem that is prominent in prior art hydrocarbon cracking to form gaseous mixtures is the poisoning effect of sulfur on the catalysts in prior use. This requires desulfurization of the feed to a sulfur content of less than about 0.2 ppm.
The steam reforming reaction is an endothermic reaction and, disirably the reaction is carried out at high temperatures. However, the prior catalysts containing nickel oxide and oxides of alkali metal as a main ingredient have a disadvantage in that, at a high temperature, e.g. about 850.degree.C., this catalyst ingredient reacts with its refractory carrier to form a solid solution, and hence rapidly becomes deactivated.
The present invention provides a process for steam reforming of various hydrocarbons without the above disadvantages.
For the purpose of finding a process for gasifying fractions heavier than naphtha, including kerosene, light oil, etc. according to steam reforming reaction, the inventors tried to utilize those fractions heavier than naphtha and found that carbon deposition on the catalyst was generally marked, and that activity of the catalyst under these circumstances lowered rapidly.
In generaly, hydrocarbons become unstable and their decomposition points become lower as the number of carbon atoms increases. Accordingly, cracking heavy hydrocarbons is possible at a low temperature, and some prior preliminary investigations of cracking these materials have been made. The equilibrium composition in the steam reforming reaction at a low temperature, contains methane in a large proportion, and this is hence unsuitable when hydrogen gas of a high purity is desired. Moreover, precipitation of carbonaceous materials cannot be avoided.
As an example of these prior investigations, it has been reported that the quantity of methane is 31.7 percent in a dry gas which is in a thermodynamic equilibrium state at 500.degree.C., and 30 kg/cm.sup.2 G and the proportion of steam to carbon in the process is 3.
The following experiments are given to illustrate basic relationships. To illustrate steam reforming reaction of hydrocarbons at a temperature above 800.degree.C., a mixture of hydrocarbons and steam was first passed through a hollow tube heated externally. Experimental conditions were as follows:
Hydrocarbon supplied: Kuwait crude oil fractions having end point of 170.degree.C. Quantity of hydrocarbon: 6.94 g/min. Molar ratio of steam to carbon: 4.0 Reaction temperature 1,100.degree.C. Quantity of gas produced: 23.2 Nl/min. Composition of produced gas (vol. %): H.sub.2 65.3, CH.sub.4 9.6, CO 18.4, CO.sub.2 6.5, C.sub.2 H.sub.4 0.1, C.sub.2 H.sub.2 0.1 Rate of gasification of carbon: 75.0%
The results of observations made during the progress of the reaction within the reaction zone are shown in FIG. 1 of the accompanying drawings.
In FIG. 1, the distance between the inlet and outlet of the reaction zone is shown on the abscissa in equal increments, and the rate of gasification of carbon is shown on the left ordinate on a log scale. The change in the rate of gasification of carbon is shown by a broken line in the figure.
Rate of gasification of carbon as used herein is defined as follows: ##EQU1##
In the same manner, selectivity is shown on the right ordinate on a log scale. The change in selectivity of the gas producing reaction for the respective components in the resulting gaseous mixture obtained from the carbon in the supplied hydrocarbons is shown by solid lines.
Considering the basic relationships as shown in FIG. 1, it is noted that the uppermost broken line showing the rate of gasification of carbon decreases to a point near the outlet of the reaction zone, and this suggests that the supplied hydrocarbons are not reacted with steam but merely thermally cracked into ethylene or acetylene. If residence time is extended in order to reduce the residual hydrocarbons, precipitate of carbonaceous substance is increased in amount.
This is clearly indicated by the curves showing selectivities of acetylene and ethylene, respectively, and also by the curves showing selectivities of steam, carbon monoxide and carbon dioxide in FIG. 1.
In the production of olefins by the thermal cracking of hydrocarbons, a method is used in which the residence time in the reaction zone is short so as to avoid precipitation of carbon. However, if the object is the production of hydrogen, the residence time cannot be shortened without causing an increase in the quantity of non-reacted materials or intermediate products.
Attempts therefore to attain a process for gasification situated between the conventional catalytic steam reforming process and the prior non-catalytic partial oxidizing process from the standpoint of the varieties of starting hydrocarbons and reaction temperatures, revealed that any significant increase in rate of gasification of carbon cannot be achieved in the reaction carried out in a reactor having no catalyst as above described.